In many types of mobile communications equipment, including wireless communications equipment such as a cellular telephone, user inputs are entered through the operation of a keypad. The keypad is conventionally constructed in the form of a crosspoint switch, wherein individual keys occupy respective nodes of an array that constitutes the keypad. Information from and to the user is conveyed through the keypad array in the course of keypad scanning procedure. The scanning procedure most frequently incorporates a debounce algorithm that is calculated to validate perceived key activations. The keypad is perhaps the most general user input that is recognized by the communications equipment. (Those skilled in the art will understand that embodiments of the invention are applicable to myriad forms of communications equipment, of which the cellular telephone is but a salient example. Accordingly, the term “equipment” or “communications equipment” is intended here to include not only cellular telephones, but all types of user-operated equipment that comprise a user input device and that operate concurrently with the receiving or transmitting of a signal such as a radio frequency (RF) signal). In general, a keypad scanning procedure requires that outputs from the baseband be passed through the keypad array and subsequently returned to the baseband to determine whether any key has been depressed, or otherwise “selected” by the user.
In the context of an integrated transceiver, such as a single chip communications terminal (i.e., a device that is designed and implemented so that both RF (radio frequency) and baseband functions and circuitry reside on the same integrated circuit (IC)), the suppression of digital switching noise and the minimization of current loops are design aspirations. Conventional keypad scanning techniques distribute current from baseband circuitry to the uncontrolled large area occupied by the keypad array, and then back to the baseband. The activation of such large current loops during the presence of RF activity creates the potential for the keypad activity to interfere with the radio performance. As performed in this manner, keypad scanning is antithetical to the objective of limiting current loops.
Irrespective of the manner in which it is accomplished, the elimination of current loops during intervals when the RF transceiver is in operation (sometimes referred to herein as a “radio event”) persists as a prevailing design objective in an integrated transceiver. The digital interference generated by the keypad scanning activity, which will be responsible to power up large areas of a system circuit board, tends to adversely affect the performance of the RF transceiver. Keypad arrays are commonly designed to apply power to all the array outputs until a keypress is detected, in the course of a keyscan process, for example.
Accordingly, there persists a need, particularly in integrated single chip communications terminal architectures, to isolate the keypad scanning procedure from radio events, e.g., during the reception or transmission of an RF signal. In addition, in providing radio event isolation during a keypad scanning procedure, deference must be paid to existing keypad scanning techniques. In particular, it is preferred that enhancements effecting radio event isolation during keypad scanning be downward compatible with equipments that are configured to include separate (i.e., non-integrated) transceiver and baseband blocks. For example, it is desired that keypad debouncing be accomplished in substantially the prevailing manner.